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Chevy Volt Analysis III

Chevy Volt Analysis III

The goalThe goal is to compare conventional internal combustion vehicles (ICEV), battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug in hybrids vehicles (PIHs). The hope, of course, is to provide an apples to apples comparison, but that’s nearly impossible do. Its not fair to compare a maximized design to an un-maximized design; it would be a lot like comparing a bottle rocket to a Boeing 747 and deciding that the airplane is therefore, more advanced than a Saturn V. I am going to explain the design details which are congruent to all vehicles so that we can understand the compromises inherent to each car.

Because ICEVs, BEVs, HEVs, and PIHs have such radically divergent strengths and weaknesses, the best comparison will be a limited one. I will use the following criteria. Obviously, vehicles can me made which do not fit these criteria, but my desire to compare the sort of vehicles which are immediately obvious to Johnny Sixpack to be, in fact, a car and not some sort of laboratory freak that does great things, but has a snowball in hell’s chance of ever being a real product this decade. I’m going to go into quite a bit of detail because only by comparing the vital subsystems can we make a fair comparison of such different cars.

Definition of a car

For our purposes, a car is vehicle that meets the following criteria: Has 4 wheels, seats (at least) 2 in a side by side arrangement, goes at least 60 MPH, has creature comforts comparable to other cars that similar drivers drive, and has a range of at least 1 hour or 50 miles, and can be supported by the existing maintenance and fueling infrastructure. Any car that does not meet the above definition will not be seen as “real” car by anyone but enthusiasts.

Design elements relevant to all cars

All cars need the following, power train, suspension, chassis, body, and fuel storage.

Power train:

Power train is the method by which potential energy stored in the fuel is converted in kinetic motion which moves the car. Rotary motion is put into wheel/s which cause the car to move. Some components are part of more than one system. The axles and wheels, for instance have a function to both the suspension and the power train. Each of the 4 cars in consideration uses relatively divergent methods to do this which will be explained later, but for now I will I will say this: ICEVs use an internal combustion engine attached to a transmission and differential (a differential is device which splits power between the two driven wheels). BEVs use an electric motor, which may be joined to the differential directly, or may have simple transmission. BEVs can also use an electric motor in 2 or more wheels called a hub motor. (This has serious problems with unsprung weight, as will be explained below). HEVs use most of the components of both a ICEV and a BEV. PIHs are HEVs that can be charged from conventional household current as well as from the ICE that is carried.

Suspension:

Suspension reflects a series of compromises on mutually exclusive conditions. The suspension is the foundation of the car in almost every way. Heavy suspension demands heavy cars, light suspension needs only a light car. Further for reasons explained below, every 1 lb in the suspension is roughly analogous to 100 lbs on the car itself.

The most basic component of the the suspension is the tire/wheel. If the tire is high pressure and narrow in relation to its hight (like a bike tire) it will convey power to the ground with high efficiency but lack traction (ie, if the rotary force changes quickly, like a stab on the throttle or brake peddle, the tire will skid. It will also skid if the car goes into a turn to quickly as the centrifugal force will quickly be larger than the tires ability to resist it). If the tire is low pressure and chubby (like tractor tires) tire will dig into the road, allowing rapid acceleration, braking, and turns. But that “digging into the road” eats energy, lowing efficiency. Also, if the tire is too chubby it will roll perpendicular to the movement of the wheel in a turn, blowing out. (Much like a runner turning his ankle in a tight turn.)

The wheel must be connected solidly to the vehicle to prevent it falling off in hard turns, acceleration or braking. However, the heavier the wheel is, the greater inertia it gathers when it hits a bump. The greater the inertia in the wheel when it hits road irregularities, the heavy the shock absorber must be to absorb the shock, this heavier shock must in turn attach to proportionally heavy chassis. A heavy chassis, of course, demands heavier suspension to suspend its bulk. This can be a vicious circle and is the number one reason that car models tend to get heavier and heavier every year.

Brakes should be as effective as possible however, again if the brakes are too heavy they contribute to the weight problem mentioned above. Brakes can be mounted on the axle toward the center of the car rather than in the wheel, which effectively makes the brakes chassis weight, rather than suspension weight (in the industry this is called sprung instead of unsprung weight). Of course, this is only possible with the driven wheels, since they are the only ones connected by a rotating axle. Also, brakes are a friction machine. They convert the energy stored in the moving mass of the car into heat. The heavier the car, the more heat and the bigger the brakes have to be, the heavier the brakes, the heavier the suspension and the heavier the car. Regardless, brakes must be cooled to work correctly. The hotter they get, the less heat they can absorb and the less braking they can do. Brakes located inboard rather than outboard have a hard time getting enough cool air flowing by to cool properly.

The axle is part of the suspension as well. The purpose of the axle is to transmit turning motion to the wheels. The axle must be heavy enough to do the job, but not so heavy as to weigh down the suspension. However, if it is too light, it will break, or worse, deform under load in a manner that causes the failure of the more expensive parts in connects. If the axle drives the rear wheels it can be “solid” (ie the wheels move up and down together). This has higher unsprung weight than the wheels moving up and down independently, which should have all sorts of negative effects. On anything but race cars, solid axles rear wheel drive is actually lighter over all, though heavier in unsprung weight. The lightness over all spells a lighter total weigh, which means less energy for a given amount of acceleration.

As I said, suspension design represents a series of compromises the goals which are mutually exclusive. Good suspension design is the foundation of good car design. However, the best suspension is only as good as the strength and accuracy of the chassis it is attached to.

The chassis:

The purpose of the chassis to relate the loads to each other that are placed on the car from the following sources: power train load, weight of components, weight of fuel storage, inertia of suspension components, aerodynamic load from the body, and weight of payload/passengers. Further, the chassis must protect the passengers in the event of collision and provide a set of attachment points for the body. Again this involves a set of mutually exclusive and interconnected goals. The chassis must be strong. If its heavy, it must be stronger to carry its own weight. Also, if it is heavy, it demands heavier components, which having raised the weight of the car demands a stronger and heavier power train.

Suspension loads hit the car from individual wheel’s suspension points, but also in ways which are not immediately transparent to the casual observer. The car must be resist beam load which tends to bend the car in the middle between the two axles. It must also resist torque load which is twisting load between the axles (as if a giant took the two axles in his hands and tried to twist the car in half). The power train also puts in a torque load, but on the axis of the drive shaft. The problem of weight of components is usually fairly straight forward, however, collision resistance is very tricky. Large heavy components do not decelerate well. In an accident, when the vehicle stops, very very quicky (over 200 negative Gs for a moment) the engine will attempt to continue in the direction the car was going. Since most accidents are from the front, this is one of the reasons that heavy engines are rarely mounted BEHIND the passengers. The engine will tear through the back seat and turn the passengers into raspberry jam in far less time than it takes to talk about it. If the engine is mounted in front, its forward motion is far better checked by having to go through the front of one car and the back of another before it can squish a person.

The body:

The body of the car has several purposes. The first is to protect the other components/passengers/payload from the elements. The second is ensure efficient airflow around the car. The third is to make the car look pretty enough to sell. Thought the first two are obviously more important from a engineering standpoint, if no one buys a design, it will never be on the highway, regardless of engineering quality. Furthermore, design costs money, and people will not pay for engineering they don’t know they need. Pretty, or at least salablely attractive is (sadly?) totally necessary.

Fuel storage:

Obviously in a ICEV this is going to be a liquid fuel tank, be it diesel, gasoline, or ethanol. Perhaps in future it might be an ethanol or hydrogen tank. In a BEV its going to be the battery pack. In a HEV or PIH its going to be have both a fuel tank and battery pack

The Cars

ICEVs

The defining characteristic of an ICEV is the internal combustion engine (ICE). The purpose of the engine is combine hydrocarbon fuels (usually gasoline or diesel) with oxygen (in the air) to make heat. This heat raises the pressure of said air, which is expanded to make power. Most of the heat in an ICE is lost. To be affordable engines must be made of affordable materials, which means that a lot of the heat they make must be gotten rid of before it can damage the engine components. In a municipal power plants and large ships, purchase price isn’t that important compared to fuel costs over 25 – 50 years, so such engines are made with absolute premium materials. This allows for around 40% efficiency. A car engine will make around 25% on a good day. Further, the car engine can only make this efficiency in very narrow range of RPMs, which is why a car requires a transmission.

The transmission allows the engine to make power efficiently, while changing the input RPMs to the wheels. Transmissions always have losses, as do tires. In the average car after the energy in the engine has been created and must be lead in a torturous path through the engine, transmission, differential, axle/s, and tires, reducing totally efficiency can to as low as 15%. However, that is for the average American car, with a big lazy engines turning sloppy automatic transmissions, through differentials designed to be quiet rather efficient, and soft, chubby tires designed to keep drivers’ butts pampered rather than have maximum traction for minimum energy loss. In real, existing vehicles, 25% efficiency is attainable.

BEVs

Battery electrical vehicles are simple in theory: a battery and a motor. The battery replaces both the gas tank and the cylinders of the engine in an ICEV, the motor replaces the rest of the engine. In reality, they are not so simple. Batteries are NOT primarily electrical machines, they are chemical reactors which produce electricity as a result of the chemical reaction. This reaction is reversible. When a battery is charged it changes the chemical relationship of 2 or more chemicals. The creates potential energy. When these chemicals convert back to their resting state, they release electrons.

Lead acid batteries, for example, are the old standby for electrical storage. Lead acid batteries are not really great at anyone thing, but represent one of the best compromises to the (again) mutually exclusive goals that electrical storage technology must meet. The important things for a battery are energy/weight, energy/size, power to weight, charge or discharge efficiency, self discharge rate (per month), and cycle limit (how many times it can charged and discharged)

(Table 1 not representable in Y360) As you can see, lead acids are very poor performers in many ways. Lithium polymers beat lead acids batteries in almost every way. Lead acids do have phenomenal rates of discharge, which is one of the reasons that they are still used in submarines and telecommunications backup systems. The other is price. $0.24 per amp/hr for lead acid. $3.25 per amp hour for li-pos. That’s 1350% A battery pack for a BEV (lead acid) will cost around $3000. Or around $40,500 for li-po. Just for the battery. Ouch.

Obviously, there is a huge weight penalty for battery packs, even good ones like lithium polymer. A kilo of gasoline contains over 5000% more energy than a kilo of li-po cell. A kilo of gasoline contains 46,900% more energy than a kilo of lead acid battery. Either the percentage of fuel storage weight to vehicle weigh goes up astronomically, or the amount of fuel/energy carried goes down astronomically, which in turns reduces range.

Further, while electrical motors can generate torque from standstill, and ICEs can’t, electric motors operate at peak efficiency in narrow range, just like ICEs. This means that electric motors demand transmissions as well, or the already enormous battery pack must be still larger due to the lack of efficiency. Also, if there is one electric motor, but two driven wheels, a BEV will still require a differential. Electric motors are often quoted as being lighter than a gas engine of equal horsepower. This is not strictly true. A very well designed electric motor functioning in an ideal environment will always appear to be better than a mass market engine designed to run under real conditions. But that is hardly a fair comparison.

A real world example is the D&D Motor Systems model# ES-31B. This motor has a peak rating of 50hp, and continuous rating of 18hp. It weighs 83 lbs. The Suzuki Swift/Geo Metro/Chevy metro engine (a 997cc triple) also had a peak rating of 50hp and a continuous rating of around 18hp. It weighed less than 75 lbs. Yet more telling is that an motor is merely an energy exchange device. It turns one form of energy into another, in this case electrical energy into mechanical energy. An engine is chemical reactor and motor. It creates the conditions for chemical reaction, reacts the chemicals, converts the energy from one form to an other and disposes of the reacted chemicals. All in one device!

The oft quoted “near 100% efficiency” of BEVs versus the 15% efficiency of ICEVs is a ridiculous distortion of the facts. An ideal electrical motor under ideal conditions will put out 97% at the shaft, not at the tire. A BEV has exactly the same drive train losses as ICE going through the transmission and differential. If the transmission and differential of an ICE take 10% of the top, so do the transmission and differential of a BEV. The differential can be skipped if each wheel has a motor, but this means that each wheel must have a transmission as well. If the motor is mounted in the wheel, 2 to 4 small motors weigh more than 1 big motor with the same sum power, and contribute to greater unsprung weight.

Worse still, electric motors are most efficient when connected directly to the battery, but a car must have a throttle to go any speed beside full. The “throttling” of DC electricity is a simple task. All that is needed is transistor, and transistors are efficient devices. However, like electric motors, and internal combustion engines, transistors have certain operational parameters where they are most efficient. Also, transistors can only handle so much current. Thus the controller (the solid-state device in an a BEV which controls current flow to the motor) must be made in a fashion somewhat like a microchip, containing thousands of transistors, with each transistor carrying a portion of the load. Though transistors are efficient, the tiny loses of each when power must pass through thousands of them becomes significant. Since they must also operate at a verity of loads, real world losses are often around 15%.

So BEVs have the following disadvantages: despite very high theoretical efficiencies, real world efficiencies are often around 50%, high weight, compromised suspension and chassis design (to accommodate the the battery pack) which in turn reduces traction, road holding, and above all range.

As mentioned, battery packs are only good for a certain number of cycles of discharge/charge before needing replacement. Though a watt of power from the light company cost much less than a watt of power from the gas station, the replacement cost of batteries is offsets this savings significantly. (Engines also wear out, but engines are can be repaired. Battery packs can only be recycled.)

Advantages are few, but important. Despite the cost of batteries, electricity, and the 50% efficiency, BEVs do cost less to operate. They also have significantly less maintenance costs. Electric motors use simpler transmissions, and put a smoother, less damaging load one them. If the BEV is designed with regenerative braking (the inertia of the vehicle drives the motor as a generator, returning energy to the battery pack) the mechanical brakes will last much longer. (Though regenerative braking only returns very limited amounts of power when the vehicle is decelerating from speeds of less than 30mph, it is precisely above those speeds where the most brake wear occurs.) Finally, even brushed electric motors (not the most efficient type, by any means) have less than 4 moving parts. The more efficient PMDC type can be made with a single moving part. The maintenance cost of BEVs, minus the cost of battery replacement is, in fact, almost none existent.

In conclusion, the BEV is still (after over a century) an immature technology, suitable only for commuter vehicles. This is not quite the faint praise it sounds like. Most Americans already own two vehicles, one designated as the heavy hauler and one designated as the commuter vehicle. For two car families, the BEV can be used for around 40% of all trips. (This conflicts with the oft stated 80-90% for obvious reasons. If two cars are being used, and one is only available for 80% of the trips, then it is only available for 40% of the total family trips.) BEV cost is currently to high for most people to accept the 60% loss of functionality entailed.

HEVs

The hope of HEVs is to carry a small battery pack (small in comparison to BEV, often they are over 1000lbs.) and an ICE which can either drive the vehicle forward, or turn a generator which charges the battery. The theory behind the complexity has been hit on several times in this paper, namely that different components have different ranges of operations in which they can operate at their highest efficiency. The first car to use this indirect form of power train was in fact built in 1912 by R. M. Owen & Company (Jay Leno drives one to the studio from time to time, oddly enough). EDM locomotives have been using a similar concept since 1939.

What is new is the use of a battery of supplement power and having small, cheap, powerful computers that can manage the current in small, cheap, powerful solid-state devices. The concept is based around simple fact of physics: an object in motion tends to remain in motion. A car does not use most of its energy to sustain motion, it uses it to accelerate. Then, when the car must decelerate (brake) the energy is simply lost. That accelerate/decelerate energy cost is why one of the reasons that cars get better mileage on the highway than in town. The other is the fact that Otto cycle engines (conventional gasoline engines) are terribly inefficient at idle. A car idling uses about 50% of the fuel it uses at full load, but produces 1/50 the horsepower. That means it takes 25 times more fuel to make 1 hp at idle than it does at highway speed. If a car is sitting at a stoplight idling, fuel is being used, but mileage isn’t being racked up so average fuel mileage is decreasing. (Diesels by the way have excellent idle performance, this is one of the many reasons they have better mileage and also one of the reasons they are used in semi-tractors, which must often idle their engines to provide energy to refrigerators on refrigerated loads, as well as provide energy to light and warm the sleeper cab.)

The hybrid in its most basic form cruises on the highway with a small efficient motor, getting good mileage. When it pulls to stop, the regenerative brakes take the deceleration energy instead of simply transferring to into heat like a conventional (friction) brake. Once stopped, the computer determines that the engine has interred inefficient idle mode, and shuts the engine off while simultaneously starting the electric motor. Inside the car air conditioning, heat, and electronics continues without any interruption. Outside the car, headlights, tail lights, etc, also continue to function without interruption. When the light turns green, the computer registrars how much power the driver is asking for (by how hard the gas peddle is pushed) and checks this value against a table of values programed to give the car the best reasonable compromise between economy and performance. The car then accelerates away from the stoplight with only the electric motor running on battery power until the power table versus the economy table shows the computer that it is time to turn on the engine. The computer then decides if the most efficient use of the engines power at that thousandth of a second is to charge the battery, or drive the car more directly.

This is basically a simple process, complicated primarily because of the consumer demand that change between different modes be totally indiscernible. The advantages are increased fuel economy (under most conditions) or increased acceleration at same fuel economy.

The disadvantages are numerous and serious. First is complexity. There is no magic potion to decrease the weight and complexity of two parallel drive systems and two energy storage methods (one of which weighs 10 times more than the other to contain the same energy [ie the gas tank and the battery pack]). The only method is reduce the size of both down to supplementary systems. Since both must be supplementary, neither is truly capable of being the primary energy system. That being the case, early hybrids are known for having problems in high plains transitioning to mountains. The driver would demand standard speed on a continuous hill. The software would oblige, having no idea that the hill would go on for 100 miles, so the battery would supplement the engine’s meager power (Remember that the engine must be undersized because of the enormous weight penalty of the battery pack that must be made up for, as well as for high economy.) When the battery pack was empty, the car would have only the small engine to drag the dead weight of the generator/motor unit and battery pack. Since weight allowance (again because of the battery pack) does not allow the use of a full transmission the small engine cannot do its best work. The hill is climbed slowly, at relatively poor fuel economy.

If this makes it sound like hybrids are would get better fuel economy without the additional weight of the battery pack and parallel drive system, its somewhat true. The Honda Insight was an early hybrid, and it has been proven that the same engine (a fairly advanced piece of work in its own right) fitted with a high quality manual transmission and the battery pack removed will actually get better highway economy. However, no battery pack and generator means both no regenerative braking and no electric take off, which is the key to the good in town mileage. Again, car design represents a set of compromises between variables, many of which are competitive and mutually exclusive.

However, the second generation of hybrids has taken a different approach to these compromises, merely providing above adequate fuel economy with a noticeable, though not substantial (again due to the lack of energy density in the battery back) improvement in acceleration. This is shown in a corresponding decrease in fuel economy. Early hybrids, like the Prius and Insight achieved economies of over 50 mpg. Second generation hybrids often struggle to get 35mpg. However, since the electrical element of the car is only serving as a supplement to the gasoline powered drive train, the vehicle has the expected power on very long grades. Also note worthy, second generation hybrids often have lower city than highway mileage, despite the fact that the gain in city mileage is the only technological justification for the dual power plant system.

The conclusion of HEVs: they represent a unique and technology daring method of design compromise, which like BEVs is clearly hamstrung by the lack of a better energy storage system. The technology may be immature, but realistically, with over 100 years of hybrid drive in ships, 70 years in submarines, and 60 years in locomotives, it seems more likely that the technology is simply not totally appropriate to consumer demands being placed on it. Further analysis of hybrid cars seems to point to the fact that all benefits being gained are the result of the maximization of design in the components that hybrids share with ICEVs and not unique nature of components special to the hybrid.

PIH

Plug in hybrids seek to combine the best (?) aspects of the HEV with best aspects of the BEV. Essentially, its as if someone realized that a small, maximized design with a 1000 lb battery pack was about 95% of the way to a BEV anyway. In fact, there are currently companies already modifying out-of-warranty hybrids into PIH.

I’ll use the example of the Chevy Volt. The car holds a large enough charge to go around 40 miles on the battery alone (which is charged at home or at work with a simple charger), or get 50 MPG highway mileage. If you need to go, say 60 miles, then the first 40 miles will be at zero fuel usage, and the last twenty miles will be at 50 MPG. 20 miles at 50 miles per gallon uses 0.4 gallons of gas. 0.4 gallons of gas to go 60 miles is 150 MPG, so even if your drive was 60 miles you still get 150 MPG.

This represents a excellent advance over BEV and HEV technology in functionality. Due to li-ion cells, and the need to only contain enough energy for 40 miles, the battery pack is a more manageable size and weight. Since around 78% of all trips are under 40 miles this means the car will operate in BEV mode often. With the trips of 100 miles offering a laudable 90 miles per gallon, nearly all trips but cross country excursions will benefit, and even those will take place at 50 miles per gallon.

Also beneficial is the fact that while many gains of BEV are made the single largest concern for many buyers (long charge times and/or lack of charging infrastructure) can be side stepped.

The disadvantages remain inherent to the HEV concept though somewhat reduced, namely, the expense of building two power trains, the expensive of maintaining two power trains, the compromised packaging resulting from having to store energy in a relatively low density medium. Further, the highway mileage of 50 MPG does not, ipso facto, point to design maximization. Indeed it might point the opposite direction, small cars of limited speed (which the Volt is) have been getting 50 MPG since the post-war boom of the late 1940’s.

Also pointing this direction, while the Volt offers outstanding improvements in the HEV power train (compared to first and second generation hybrids) there has been almost no improvement whatsoever to the ICE which represents at least ½ the drive train mass. If you recall from earlier, light-and-power plants and maritime diesels achieve efficiencies of 40% on a regular basis. This is at least a 200% improvement over conventional ICE car engines. This technology is not only scalable, but has already been scaled, analyzed, and executed by the auto performance after-market. Simple economic analysis would suggest improving existing technology with existing supportive technology will be more cost effective than attempting to support intrinsically limited technology with a newer and less developed, previously unscaled technology.

Again pointing to a lack of design maximization, the Volt does not weigh less than the cars it competes with. It is not more aerodynamic than the cars it competes. The lack of true transmission means that when the house charged 40 miles is up, the vehicle will always take a higher drive train loss than 1930’s luxury car. (Generator to motor couplings always exhibit higher losses than a drive shaft of the same length. Good manual transmissions, however, are 97% efficient.) In fact, the entire tool box of standard automotive efficiency improving modifications has been virtually ignored.

It is for this reason that I believe the Volt, though certainly better than many of the truly awful cars currently available, is not a real attempt to solve the problem of fuel economy from an engineering standpoint, but an attempt to offer the public limited improvement, congruent with techno popculture buzzwords and GM’s long standing practice of planned obsolescence. I believe that while GM is collecting meaningful experience and data, and may even produce the car, that the Chevy Volt has much more in common with the GMR (GM’s almost Wankle of the late 70’s) than it does with the EV1.

When GM made the EV1 did not merely seize the car from its lessees as it had the full legal right and financial obligation to do. GM was so repulsed by the idea of BEVS that it totally destroyed the cars and fired every person who sold them or engineered them. The Volt may be a fine little car, but ultimately, GM’s first obligation is not to its customers but its stock holders. GM can do better, but will not.